Stress impairs new but not established relationships in seasonally social voles

Comparative analysis of gonadal hormone receptor expression in the postnatal house mouse, meadow vole, and prairie vole brain

The socially monogamous prairie vole (Microtus ochrogaster) and promiscuous meadow vole (Microtus pennsylvanicus) are closely related, but only prairie voles display long-lasting pair bonds, biparental care, and selective aggression towards unfamiliar individuals after pair bonding. These social behaviors in mammals are largely mediated by steroid hormone signaling in the social behavior network (SBN) of the brain. Hormone receptors are reproducible markers of sex differences that can provide more information than anatomy alone and can even be at odds with anatomical dimorphisms. We reasoned that behaviors associated with social monogamy in prairie voles may emerge in part from unique expression patterns of steroid hormone receptors in this species, and that these expression patterns would be more similar across males and females in prairie than in meadow voles or the laboratory mouse. To obtain insight into steroid hormone signaling in the developing prairie vole brain, we assessed expression of estrogen receptor alpha (Esr1), estrogen receptor beta (Esr2), and androgen receptor (Ar) within the SBN, using in situ hybridization at postnatal day 14 in mice, meadow, and prairie voles. We found species-specific patterns of hormone receptor expression in the hippocampus and ventromedial hypothalamus, as well as species differences in the sex bias of these markers in the principal nucleus of the bed nucleus of the stria terminalis. These findings suggest the observed differences in gonadal hormone receptor expression may underlie species differences in the display of social behaviors.

Acute Stress Modulates Social Approach and Social Maintenance in Adult Zebrafish

Stress alters social functioning in a complex manner. An important variable determining the final effects of stress is stressor intensity. However, the precise relationship between stressor intensity and social behavior is not well understood. Here, we investigate the effects of varying acute stressor intensity exposure on social behavior using adult zebrafish. We first establish a novel test using adult zebrafish that allows distinguishing fish's drive to approach a social cue and its ability to engage and maintain social interaction within the same behavioral paradigm. Next, we combined this test with a new method to deliver an acute stress stimulus of varying intensities. Our results show that both social approach and social maintenance are reduced in adult zebrafish on acute stress exposure in an intensity-dependent manner. Interestingly, lower stress intensity reduces social maintenance without affecting the social approach, while a higher stress level is required to alter social approach. These results provide evidence for a direct correlation between acute stressor intensity and social functioning and suggest that distinct steps in social behavior are modulated differentially by the acute stress level.

Selectivity and Sociality: Aggression and Affiliation Shape Vole Social Relationships

The formation of selective social relationships is not a requirement of group living; sociality can be supported by motivation for social interaction in the absence of preferences for specific individuals, and by tolerance in place of social motivation. For species that form selective social relationships, these can be maintained by preference for familiar partners, as well as by avoidance of or aggression toward individuals outside of the social bond. In this review, we explore the roles that aggression, motivation, and tolerance play in the maintenance of selective affiliation. We focus on prairie voles (Microtus ochrogaster) and meadow voles (Microtus pennsylvanicus) as rodent species that both exhibit the unusual tendency to form selective social relationships, but differ with regard to mating system. These species provide an opportunity to investigate the mechanisms that underlie social relationships, and to compare mechanisms supporting pair bonds with mates and same-sex peer relationships. We then relate this to the role of aggression in group composition in a comparative context.

Sex differences in the reward value of familiar mates in prairie voles

The rewarding properties of social interactions facilitate relationship formation and maintenance. Prairie voles are one of the few laboratory species that form selective relationships, manifested as "partner preferences" for familiar partners versus strangers. While both sexes exhibit strong partner preferences, this similarity in outward behavior likely results from sex-specific neurobiological mechanisms. We recently demonstrated that in operant trials, females worked hardest for access to familiar conspecifics of either sex, while males worked equally hard for access to any female, indicating a sex difference in social motivation. As tests were performed with one social target at a time, males might have experienced a ceiling effect, and familiar females might be more relatively rewarding in a choice scenario. Here we performed an operant social choice task in which voles lever-pressed to gain temporary access to either the chamber containing their mate or one containing a novel opposite-sex vole. Females worked hardest to access their mate, while males pressed at similar rates for either female. Individual male behavior was heterogeneous, congruent with multiple mating strategies in the wild. Voles exhibited preferences for favorable over unfavorable environments in a non-social operant task, indicating that lack of social preference does not reflect lack of discrimination. Natural variation in oxytocin receptor genotype at the intronic single nucleotide polymorphism NT213739 was associated with oxytocin receptor density, and predicted individual variation in stranger-directed aggressive behavior. These findings suggest that convergent preference behavior in male and female voles results from sex-divergent pathways, particularly in the realm of social motivation.

Social selectivity and social motivation in voles

Selective relationships are fundamental to humans and many other animals, but relationships between mates, family members, or peers may be mediated differently. We examined connections between social reward and social selectivity, aggression, and oxytocin receptor signaling pathways in rodents that naturally form enduring, selective relationships with mates and peers (monogamous prairie voles) or peers (group-living meadow voles). Female prairie and meadow voles worked harder to access familiar versus unfamiliar individuals, regardless of sex, and huddled extensively with familiar subjects. Male prairie voles displayed strongly selective huddling preferences for familiar animals, but only worked harder to repeatedly access females versus males, with no difference in effort by familiarity. This reveals a striking sex difference in pathways underlying social monogamy and demonstrates a fundamental disconnect between motivation and social selectivity in males-a distinction not detected by the partner preference test. Meadow voles exhibited social preferences but low social motivation, consistent with tolerance rather than reward supporting social groups in this species. Natural variation in oxytocin receptor binding predicted individual variation in prosocial and aggressive behaviors. These results provide a basis for understanding species, sex, and individual differences in the mechanisms underlying the role of social reward in social preference.

Familiarity and Mate Preference Assessment with the Partner Preference Test

In contrast to traditional laboratory animals, prairie voles form socially monogamous partnerships in the wild and exhibit lasting social preferences for familiar individuals-both mates and same-sex peers-in the laboratory. Decades of research into the mechanisms supporting pair bonding behavior have made prairie voles an important model organism for the study of social relationships. The partner preference test is a laboratory test of familiarity preference that takes place over an extended interval (typically 3 hr), during which test subjects can directly interact with conspecifics and often engage in resting side-by-side contact (i.e., huddling). The use of this test has enabled study of the neural pathways and mechanisms involved in promoting or impairing relationship formation. The tendency to form partner preferences is also used as a behavioral indicator of the effects of early life experiences and environmental exposures. While this test was developed to assess the extent of social preference for mates in prairie voles, it has been adapted for use in other social contexts and in multiple other species. This article provides instructions for conducting the classic partner preference test, as well as variations including same-sex "peer" partner preference tests. The effects of several protocol variations are examined, including duration of cohousing, separation interval, use of tethers versus barriers, linear versus branched apparatus configuration, and duration of the test. The roles of social variables including sex of the focal individual, sex of conspecifics, reproductive state, and use of the test in other species are then considered. Finally, sample data are provided along with discussion of scoring and statistical analysis of partner preference tests. © 2021 Wiley Periodicals LLC. Basic Protocol: Partner preference test Support Protocol: Behavioral scoring.

A Neuroscientist's Guide to the Vole

Prairie voles have emerged as an important rodent model for understanding the neuroscience of social behavior. Prairie voles are well known for their capacity for pair bonding and alloparental care. These behavioral phenomena overlap with human social behavior but are not commonly observed in traditional rodent models. In this article, we highlight the many benefits of using prairie voles in neuroscience research. We begin by describing the advantages of using diverse and non-traditional study models. We then focus on social behaviors, including pair bonding, alloparental care, and peer interactions, that have brought voles to the forefront of social neuroscience. We describe many additional features of prairie vole biology and behavior that provide researchers with opportunities to address an array of research questions. We also survey neuroethological methods that have been used with prairie voles, from classic to modern techniques. Finally, we conclude with a discussion of other vole species, particularly meadow voles, and their own unique advantages for neuroscience studies. This article provides a foundation for researchers who are new to working with voles, as well as for experienced neuroscientists who want to expand their research scope. © 2021 Wiley Periodicals LLC.

The call of the wild: using non-model systems to investigate microbiome-behaviour relationships

On and within most sites across an animal's body live complex communities of microorganisms. These microorganisms perform a variety of important functions for their hosts, including communicating with the brain, immune system and endocrine axes to mediate physiological processes and affect individual behaviour. Microbiome research has primarily focused on the functions of the microbiome within the gastrointestinal tract (gut microbiome) using biomedically relevant laboratory species (i.e. model organisms). These studies have identified important connections between the gut microbiome and host immune, neuroendocrine and nervous systems, as well as how these connections, in turn, influence host behaviour and health. Recently, the field has expanded beyond traditional model systems as it has become apparent that the microbiome can drive differences in behaviour and diet, play a fundamental role in host fitness and influence community-scale dynamics in wild populations. In this Review, we highlight the value of conducting hypothesis-driven research in non-model organisms and the benefits of a comparative approach that assesses patterns across different species or taxa. Using social behaviour as an intellectual framework, we review the bidirectional relationship between the gut microbiome and host behaviour, and identify understudied mechanisms by which these effects may be mediated.

Comparative Assessment of Familiarity/Novelty Preferences in Rodents

Sociality-i.e., life in social groups-has evolved many times in rodents, and there is considerable variation in the nature of these groups. While many species-typical behaviors have been described in field settings, the use of consistent behavioral assays in the laboratory provides key data for comparisons across species. The preference for interaction with familiar or novel individuals is an important dimension of social behavior. Familiarity preference, in particular, may be associated with more closed, less flexible social groups. The dimension from selectivity to gregariousness has been used as a factor in classification of social group types. Laboratory tests of social choice range from brief (10 minutes) to extended (e.g., 3 hours). As familiarity preferences typically need long testing intervals to manifest, we used 3-hour peer partner preference tests to test for the presence of familiarity preferences in same-sex cage-mates and strangers in rats. We then conducted an aggregated analysis of familiarity preferences across multiple rodent species (adult male and female rats, mice, prairie voles, meadow voles, and female degus) tested with the same protocol. We found a high degree of consistency within species across data sets, supporting the existence of strong, species-typical familiarity preferences in prairie voles and meadow voles, and a lack of familiarity preferences in other species tested. Sociability, or total time spent near conspecifics, was unrelated to selectivity in social preference. These findings provide important background for interpreting the neurobiological mechanisms involved in social behavior in these species.

The role of dopamine signaling in prairie vole peer relationships

Dopamine signaling mediates the formation of some types of social relationships, including reproductive pair bonds in the socially monogamous prairie vole (Microtus ochrogaster). In addition to these pair bonds with mates, prairie voles demonstrate selective preferences for familiar same-sex peers. The dependence of peer relationships on dopamine signaling has not been tested, and the mechanisms supporting these relationships may differ from those underlying pair bonds. We examined the effects of pharmacological manipulations of dopamine signaling on peer partner preference and socially conditioned place preference in female prairie voles. Haloperidol blockade of dopamine receptors at multiple doses did not alter selective preferences for familiar same-sex partners, suggesting that dopamine neurotransmission is not necessary for the formation of prairie vole peer relationships, unlike mate relationships. Dopamine receptor agonist apomorphine facilitated peer partner preferences under conditions normally insufficient for partner preference formation; however, in the absence of effects from blockade, it is difficult to distinguish between a role for dopamine in partner preference formation and the generally rewarding properties of a dopamine agonist. Prairie voles exhibited socially conditioned place preferences for new but not long-term same-sex peers, and these preferences were not blocked by haloperidol. These results suggest that prairie vole peer relationships are less dependent on dopamine signaling than pair bonds, while still being rewarding. The data support distinct roles of dopamine and motivation in prairie vole peer relationships relative to mate relationships, suggesting that reproductive bonds are mediated differently from non-reproductive ones.

Stress in groups: Lessons from non-traditional rodent species and housing models

A major feature of life in groups is that individuals experience social stressors of varying intensity and type. Social stress can have profound effects on health, social behavior, and ongoing relationships. Relationships can also buffer the experience of exogenous stressors. Social stress has most commonly been investigated in dyadic contexts in mice and rats that produce intense stress. Here we review findings from studies of diverse rodents and non-traditional group housing paradigms, focusing on laboratory studies of mice and rats housed in visible burrow systems, prairie and meadow voles, and mole-rats. We argue that the use of methods informed by the natural ecology of rodent species provides novel insights into the relationship between social stress, behavior and physiology. In particular, we describe how this ethologically inspired approach reveals how individuals vary in their experience of and response to social stress, and how ecological and social contexts impact the effects of stress. Social stress induces adaptive changes, as well as long-term disruptive effects on behavior and physiology.

Female degus show high sociality but no preference for familiar peers

Group-living animals vary in social behavior across multiple dimensions, including in the selectivity of social interactions with familiar versus unfamiliar peers. Standardized behavioral tests can be used to tease apart different dimensions of behavior. These serve a dual function-on one hand, helping to isolate behavioral factors that may support collective behavior in natural habitats, and, on another, providing a basis for comparative approaches to understanding physiological mechanisms of behavior. Degus (Octodon degus) are South American caviomorph rodents that nest and forage in groups with relatively low genetic relatedness. Flexibility in group membership is likely supported by gregariousness toward strangers, but the relative preference for strangers compared with familiar individuals has not been systematically tested. We assessed the specificity of social preferences in female degus using a same-sex partner preference test. Degus huddled extensively with both familiar and unfamiliar peers, with no average preference for one over the other. Detailed analysis of social interactions demonstrated an effect of familiarity on social investigation and aggressive behaviors, indicating that degus distinguished between familiar and unfamiliar conspecifics, even though it did not impact huddling. This behavioral profile is thus far unique to degus; in similar tests, meadow and prairie voles exhibit strong partner preferences for known peers, while mice exhibit low social huddling and spend relatively less time in social chambers. Understanding how group-living species differ in specific aspects of social behavior such as familiarity/novelty preference and propensity for social contact will offer a foundation to interpret differences in neural systems supporting sociality.

Comparative role of reward in long-term peer and mate relationships in voles

This is a contribution to SI: SBN/ICN meeting. In social species, relationships may form between mates, parents and their offspring, and/or social peers. Prairie voles and meadow voles both form selective relationships for familiar same-sex peers, but differ in mating system, allowing comparison of the properties of peer and mate relationships. Prairie vole mate bonds are dopamine-dependent, unlike meadow vole peer relationships, indicating potential differences in the mechanisms and motivation supporting these relationships within and/or across species. We review the role of dopamine signaling in affiliative behavior, and assess the role of behavioral reward across relationship types. We compared the reinforcing properties of mate versus peer relationships within a species (prairie voles), and peer relationships across species (meadow and prairie voles). Social reinforcement was assessed using the socially conditioned place preference test. Animals were conditioned using randomly assigned, equally preferred beddings associated with social (CS+) and solitary (CS-) housing. Prairie vole mates, but not prairie or meadow vole peers, conditioned toward the social cue. A second study in peers used counter-conditioning to enhance the capacity to detect low-level conditioning. Time spent on CS+ bedding significantly decreased in meadow voles, and showed a non-significant increase in prairie voles. These data support the conclusion that mate relationships are rewarding for prairie voles. Despite selectivity of preferences for familiar individuals in partner preference tests, peer relationships in both species appear only weakly reinforcing or non-reinforcing. This suggests important differences in the pathways underlying these relationship types, even within species.

Affiliation, Aggression, and Selectivity of Peer Relationships in Meadow and Prairie Voles

Relationships between adult peers are central to the structure of social groups. In some species, selective preferences for specific peers provide a foundation for consistent group composition. These preferences may be shaped by affiliation toward familiar individuals, and/or by aversion to unfamiliar individuals. We compared peer interactions in two vole species that form selective preferences for familiar same-sex individuals but differ in mating system. Prairie voles (Microtus ochrogaster) form pair bonds with mates and may reside in family groups. Meadow voles (Microtus pennsylvanicus) are promiscuous breeders that form communal winter groups in the wild, and exhibit greater social behavior in short day (SD) lengths in the laboratory. We characterized affiliative, anxiety-like, and aggressive interactions with familiar and novel same-sex conspecifics in meadow and prairie voles housed in summer- or winter-like photoperiods. Species differences in affective behaviors were pronounced, with prairie voles exhibiting more aggressive behavior and less anxiety-like behavior relative to meadow voles. Meadow voles housed in short (vs. long) day lengths were more affiliative and more interactive with strangers; prosocial behavior was also facilitated by a history of social housing. Prairie voles exhibited partner preferences regardless of sex or day length, indicating that selective peer preferences are the norm in prairie voles. Prairie vole females formed preferences for new same-sex social partners following re-pairing; males were often aggressive upon re-pairing. These data suggest that preferences for familiar peers in prairie voles are maintained in part by aggression toward unfamiliar individuals, as in mate partnerships. In contrast, social tolerance is an important feature of meadow vole peer affiliation, demonstrated by low aggression toward unfamiliar conspecifics, and consistent with field data on winter tolerance.

Frank Beach award winner: Neuroendocrinology of group living

Why do members of some species live in groups while others are solitary? Group living (sociality) has often been studied from an evolutionary perspective, but less is known about the neurobiology of affiliation outside the realms of mating and parenting. Colonial species offer a valuable opportunity to study nonsexual affiliative behavior between adult peers. Meadow voles (Microtus pennsylvanicus) display environmentally induced variation in social behavior, maintaining exclusive territories in summer months, but living in social groups in winter. Research on peer relationships in female meadow voles demonstrates that these selective preferences are mediated differently than mate relationships in socially monogamous prairie voles, but are also impacted by oxytocin and HPA axis signaling. This review addresses day-length dependent variation in physiology and behavior, and presents the current understanding of the mechanisms supporting selective social relationships in meadow voles, with connections to lessons from other species.

Neural Circuits Underlying Rodent Sociality: A Comparative Approach

All mammals begin life in social groups, but for some species, social relationships persist and develop throughout the course of an individual's life. Research in multiple rodent species provides evidence of relatively conserved circuitry underlying social behaviors and processes such as social recognition and memory, social reward, and social approach/avoidance. Species exhibiting different complex social behaviors and social systems (such as social monogamy or familiarity preferences) can be characterized in part by when and how they display specific social behaviors. Prairie and meadow voles are closely related species that exhibit similarly selective peer preferences but different mating systems, aiding direct comparison of the mechanisms underlying affiliative behavior. This chapter draws on research in voles as well as other rodents to explore the mechanisms involved in individual social behavior processes, as well as specific complex social patterns. Contrasts between vole species exemplify how the laboratory study of diverse species improves our understanding of the mechanisms underlying social behavior. We identify several additional rodent species whose interesting social structures and available ecological and behavioral field data make them good candidates for study. New techniques and integration across laboratory and field settings will provide exciting opportunities for future mechanistic work in non-model species.

Developmental stress and social phenotypes: integrating neuroendocrine, behavioural and evolutionary perspectives

The social world is filled with different types of interactions, and social experience interacts with stress on several different levels. Activation of the neuroendocrine axis that regulates the response to stress can have consequences for innumerable behavioural responses, including social decision-making and aspects of sociality, such as gregariousness and aggression. This is especially true for stress experienced during early life, when physiological systems are developing and highly sensitive to perturbation. Stress at this time can have persistent effects on social behaviours into adulthood. One important question remaining is to what extent these effects are adaptive. This paper initially reviews the current literature investigating the complex relationships between the hypothalamic-pituitary-adrenal (HPA) axis and other neuroendocrine systems and several aspects of social behaviour in vertebrates. In addition, the review explores the evidence surrounding the potential for 'social programming' via differential development and activation of the HPA axis, providing an insight into the potential for positive effects on fitness following early life stress. Finally, the paper provides a framework from which novel investigations could work to fully understand the adaptive significance of early life effects on social behaviours.This article is part of the themed issue 'Physiological determinants of social behaviour in animals'.

Specificity in Sociality: Mice and Prairie Voles Exhibit Different Patterns of Peer Affiliation

Social behavior is often described as a unified concept, but highly social (group-living) species exhibit distinct social structures and may make different social decisions. Prairie voles (Microtus ochrogaster) are socially monogamous rodents that often reside in extended family groups, and exhibit robust preferences for familiar social partners (same- and opposite-sex) during extended choice tests, although short-term preferences are not known. Mice (Mus musculus) are gregarious and colonial, but in brief laboratory tests of social preference they typically prefer social novelty. This preference for novel vs. familiar peers may represent a species-specific difference in social decision-making between mice and prairie voles. However, the tests used to measure preferences in each species differ markedly in duration and degree of contact, such that the behaviors cannot be directly compared. We assessed whether social preferences for novelty or familiarity differed between mice and prairie voles of both sexes when assessed with matching protocols: the sociability/social preference test (SPT) typically used in mice (short, no direct contact), and the partner preference test (PPT) used in voles (long, direct contact). A subset of voles also underwent a PPT using barriers (long, no direct contact). In the short SPT, behavior did not differ between species. In the longer test, pronounced partner preferences emerged in prairie voles, but mice exhibited no social preferences and rarely huddled. No sex differences were evident in either test. Direct physical contact was required for partner preferences in huddling time in voles, but preference for the partner chamber was evident with or without contact. Both prairie voles and mice are social, but they exhibit important differences in the specificity and extent of their social behavior. While mice are often used to study social approach and other behaviors, voles are a more suitable species for the study of selective social relationships. Consideration of these differences will be important for studies examining the neural mechanisms supporting different kinds of peer social behavior.

Septal oxytocin administration impairs peer affiliation via V1a receptors in female meadow voles

The peptide hormone oxytocin (OT) plays an important role in social behaviors, including social bond formation. In different contexts, however, OT is also associated with aggression, social selectivity, and reduced affiliation. Female meadow voles form social preferences for familiar same-sex peers under short, winter-like day lengths in the laboratory, and provide a means of studying affiliation outside the context of reproductive pair bonds. Multiple lines of evidence suggest that the actions of OT in the lateral septum (LS) may decrease affiliative behavior, including greater density of OT receptors in the LS of meadow voles that huddle less. We infused OT into the LS of female meadow voles immediately prior to cohabitation with a social partner to determine its effects on partner preference formation. OT prevented the formation of preferences for the partner female. Co-administration of OT with a specific OT receptor antagonist did not reverse the effect, but co-administration of OT with a specific vasopressin 1a receptor (V1aR) antagonist did, indicating that OT in the LS likely acted through V1aRs to decrease partner preference. Receptor autoradiography revealed dense V1aR binding in the LS of female meadow voles. These results suggest that the LS is a brain region that may be responsible for inhibitory effects of OT administration on affiliation, which will be important to consider in therapeutic administrations of OT.